The liquid crystal display element in a liquid crystal display device does not emit light itself. Liquid crystal display devices therefore have a backlight disposed behind the liquid crystal display element as a light source to illuminate the liquid crystal display element. CCF (Cold Cathode Fluorescent) lamps, in which the inner wall of a glass tube is coated with a phosphor and from which white light can be obtained, have been the leading type of light source for backlight use. With the recent striking improvements in LED (Light Emitting Diode) performance, however, the demand for backlights using LED light sources has been rapidly expanding.
Among the devices referred to as LEDs there are monochromatic LEDs and white LEDs. A monochromatic LED emits monochromatic light such as red, green, or blue light by direct emission from the LED. A white LED has a blue LED and a yellow phosphor enclosed in a package. The yellow phosphor is excited by the blue light. White light is thereby obtained from the white LED. White LEDs in particular have high emission efficiency and are effective in reducing power consumption. White LEDs are therefore widely used as light sources for backlights. ‘Monochromatic’ refers to a single color with no admixture of other colors. ‘Monochromatic light’ is light with a single narrow wavelength width.
A white LED has a wide wavelength bandwidth. For that reason, white LEDs have the problem of a narrow color gamut. The liquid crystal display element in a liquid crystal display device has internal color filters. The liquid crystal display device renders colors by using these color filters to extract the isolated spectral wavelength ranges of red, green, and blue light. With a light source having a continuous wavelength spectrum with a wide bandwidth as found in a white LED, to widen the color gamut, it is necessary to increase the color purity of the color filters of the display colors. The color filters are therefore designed to have narrow wavelength transmission bands. If the color filters have narrow wavelength transmission bands, however, then their light utilization efficiency is reduced. The reason is that there is a large amount of unnecessary light that is not used by the liquid crystal display element of the liquid crystal display device. Further problems also arise: the brightness of the display surface of the liquid crystal display element is reduced and the power consumption of the liquid crystal display device is increased.
White LEDs are particularly deficient in energy in the red spectral band from 600 nm to 700 nm. If a color filter with a narrow wavelength bandwidth is used to increase the purity in the desired 630-640 nm wavelength band, the problem arises that the amount of light transmitted is greatly reduced. Accordingly, the problem of sharply reduced brightness arises.
As a remedy to these problems, backlights using monochromatic LEDs or lasers of high color purity have recently been proposed. High color purity means a narrow wavelength band and excellent monochromaticity. Lasers in particular have extremely good monochromaticity, and their emission efficiency is also high. By using laser light sources, it has therefore become possible to offer liquid crystal display devices that display images with high brightness and a wide color gamut. By using laser light sources, it has also become possible to offer liquid crystal display devices with low power consumption.
Monochromatic LEDs and lasers emit monochromatic light. To generate white light by using monochromatic light sources, a backlight therefore needs to have red, green, and blue light sources. That is, the backlight needs to have different light sources emitting light of the three primary colors. The backlight generates white light by mixing the light emitted from these light sources. If light sources having different angular intensity distributions are used, there will be irregularities in the spatial intensity distributions of the colors on the display surface of the liquid crystal display element. When light sources of a plurality of colors are used to generate white light, these spatial intensity distribution irregularities will show up as color irregularities. That is, when mixed to generate white light, the intensity irregularities of the light of the different colors will show up as color irregularities. Light intensity irregularities are also referred to as brightness irregularities.
To solve this problem, it is necessary to increase the planar uniformity of the spatial intensity distributions. Light emitted from light sources that differ in their emitting mechanism or in the material properties of their light emitting elements will have differing divergence angles and emission efficiencies, however. For that reason, it is necessary to provide optimal means for uniformizing the spatial intensity distributions in the planes corresponding to the light sources.
In conventional backlights, color irregularities have been suppressed by using special light guide plates matched to the characteristics of the light sources. Patent Document 1, for example, proposes a backlight having a special light guide plate for the light source of each color for use in a flat display panel. This backlight for use in a flat display panel has a different light source for each color and a light guide plate corresponding to the light source for each color. The backlight generates white illumination light by additively mixing the monochromatic planar light emitted from each light guide plate. This configuration enables the structure of each light guide plate to be optimized for the characteristics of the corresponding light source. It is accordingly possible to increase the uniformity of the planar spatial intensity distribution of each color and suppress color irregularities. A ‘planar spatial intensity distribution’ is a distribution representing levels of light intensity at positions expressed in a two-dimensional plane.